One of the conditions for controlling the aerodynamic in the reaction chamber is designing a crevice volume on the surface of the piston head. The importance of the crevice volume is to contain the cool boundary layers generated as a resulting of the moving reactor piston. But this crevice volume consequently drops the end gas pressure and temperature at the end of the stroke. The CFD study of the aerodynamic effect of a piston movement in a reaction chamber is modelled using the commercial code of Ansys Fluent and assuming a 2-Dimensional computational moving mesh. A starting optimal crevice volume of 282 mm3 is used for further optimisation. This resulted in five crevice lengths of 3mm, 5mm, 7mm, 9mm and 12mm respectively. The crevice height of 5 mm was found to improve the compressed gas pressure at the end of the stroke to about 2 bar and temperature about 17.7 K and also maintained a uniform temperature field. While that of 12 mm had the least peak compressed gas pressure. This study investigates the possible means of improving the peak pressure and temperature drop in a rapid compression machine by further optimisation of the crevice volume.